All right guys I guess uh the five of us are still recuperating a little bit um including Brian since he didn't make it tonight from the Symposium It was a pretty cool experience, but um I think the whole thought behind this topic started maybe several weeks ago or a month ago in a Facebook group. Somebody asked the question like hey I have this duct and I want to change the size of my equipment and is this duct too big? will it work and this is when everyone knows Ed Janowak from Acca right He was kind of like off social media for a while and he just slipped a comment in and he said Ducks can't be too big and then he left and like chaos the bat signal went off at his house. he jumped on and typed a few things and left right. so like like this huge argument of like probably a 100 plus comments started uh including people saying that he has no clue what about airflow or or duck work in general which I thought was humorous So that was kind of the thought process behind having you gentlemen on this show since you three are probably or four I would say are probably the smartest people with airf flow that I know.

um so just before the show started we were. We were kind of starting off talking about velocity a little bit. maybe we should pick that back up again. um since you know we were kneed deep in that um Tony do uh we we were talking I know a big big thing is um you know everyone in the South says you don't want too low velocity in unconditioned space, right? Especially in Atcts when you have really hot attic air.

Uh, because you'll start, you'll change your duck gains and that's GNA have a negative impact on your s So yeah Yeah well well and well. yeah. I mean you know I I Done a lot of different types of duck modeling I haven't done a lot with very low velocity so you know I kind of have to sort of thumb through my notes and and stuff like that. Uh, one thing.

I kind of like to go back a little bit because we hear this a lot in in lectures I Just want to go back. just a little bit. is everyone keeps on talking about lamin or flow. Um now when I was in school um you know we had a criteria of what do you call laminer Flow what do you call turbulent flow and and and but my experience has been with work is everything's turbulent flow.

Um, in fact a lot of our correlations, uh, equivalent length correlations in 900 feet per minute or 700 feet per minute return stuff. you know, Um, when you plug this stuff in. Um, and by the way, the the the the general definition and you may see different numbers whether you're talking about water or oil in a pipe or or air flow ducts and and stuff like that. Uh, but generally what you're find.

so I I I I pulled from the ashray which is more specific to air flow at least in the duck stuff that they say hey, look, anything under 2,000 uh Reynolds number is lamin or flow and that's what smooth flow is. Real smooth, you know very little loss, However, very low pressure systems, very very low velocity. Tony could you tell us what is the what is a Reynolds number? Um well I I showed the formula at the bottom and and not that anyone ever wants to. you know would find this entertaining or fun to get into formulas.

Uh, but but basically one way of describing your Reynolds number. It doesn't really have a unit, but it's uh, one way of looking at it is it's a ratio of your the force of the fluid divided by the resistance that it's pushing through. So turbulent flow has a lot of strength even though you've got high velocities and you've got I'm not talking High Velocity duck work I'm just saying our typical you know, 400 feet per minute, 500 feet per minute and and more than that, you know we're We have plenty of pressure to overcome the resistance of a straight pipe duct. Okay, whether it's Flex or or PVC or metal.

Um, you know when we talk about laminar flow. Um, you're going to have smaller Reynolds numbers and that's basically just saying hey, look, the resistance is kind of significant compared to the effort or the pressure or that force that you're putting to move something in a pipe. You're moving it with very low pressure, but you're also getting some resistance. But but it's going to be smooth flow.

but turbulent flow? Uh, and I kind of have some visuals. Of course this is just from a textbook I wish I had some more uh uh, more better visuals to show, you know, but things are going to be very smooth and streamlined and it's never really like that in a in a residential duck system. You know things are going to be rough and even though you've got Eddes and these vorticities, however, the average velocities are pretty uniform. In a turbulent flow, it fills the the system really well.

Uh, it doesn't Trip Easy. Uh, so so people would say hey, look uh uh for air flow uh G or gas flow uh, you know turbulent flow would be Reynolds number greater than 10,000 So something that's you know, 10,000 times more the force of the fluid divided by the resistance force of the duck work and then laminer flow would be less than 2,000 and in between is the area of unknown. You know, we really don't have any correlations on transitional flow. In fact, we actually have very little correlations with laminar flow.

Um, and by the way, volume dampers and stuff like that they're all rated for 300 to 400 feet per minute is kind of like the lowest number you would see where you could have control and linear. uh, you know, positioning and linear flow control on dampers. So so uh Russ I Want to ask you I know like in Manual D I think a A big misconception with a lot of the fittings and the fitting selections is Um which you know I Think you know in general talking about laminer versus turbulent flow, people want to use Uh fitting selection because they think they're going to achieve laminer flow. Do you want to maybe talk a little bit about the difference in fittings and you know? plus um, you know B Basically like um, what is a bonus of using one fitting over the other? Does radius matter? Is this like you know? Does Lamer and turbant flow play into that? Um, yeah, First of all, you know, realize that Manual d a very very very very simplified approach to doing something? Uh, But when you look at the ultimate result, you know I think um, a lot of people feel like when they're doing manual JS and that they're that they're using this high-powered you know, sniper rifle.

But what they're really using is a S off shotgun and they don't need to be super super precise. You know? Um, and so what the fittings do is they just give you a resistance to airf flow and and your friction rate is like your resistance budget you know I was telling Ed the other day that by the way Ed I Heard Ed cannot walk past an ant's Nest without poking it with a stick Just so everybody know. um um it's or punching it in the throat. Yes yes Um so the the friction rate is like your is like your Um available pressure drop.

It's your budget. It's how much it's how much budget you have to work with based on how much pressure your fan gives you and and then your. the The currency is feet. It's how many feet of stuff you have before you run out of your budget, your pressure budget.

and so it's just A the different fittings are just a way to um to quantify that. that resistance In terms of feet, they they equate it to length of duct. you know. So this fitting has the same pressure drop going in and coming out as it would if that air were to go down 20 feet of duct.

You know, something like that. So it's very very very simplified. Um, as far as turbulent and and laminar flow it, no, don't even worry about that, that's you know. The other thing to realize too is just going from going from a 6inch duct to a seven-inch duct.

You know, just going up one size is a 50% increase in air flow. And so you know if we were, if we were sizing ducks in in tenth of inches. you know oh that's a we need a 7.2 inch duct. Here we need a 8.4 inch duct over there then it would be different.

You know, worrying about laminer, you know then it would be different. But the the, the the buckets we have to deal with that you know, the six inch, the seven inch, the eight and some people don't even use odd-sized duct. So it goes from a six to an 8 inch duct. you know? So it's really simplified and it's and I'm all for.

um, you know, keeping your resistance as low as possible so your equivalent lengths of your run from start to finish keep it as low as possible. But Manual D will make up for that in in by making a duct bigger to to allow for more uh, pressure drop and I think that's kind of where we start wondering. well is the is a duck too big? But if you look at manual D it gives you a maximum and minimum friction rate to work with. The The minimum is 0.006 and the maximum is 0.1 0.18 And if you stay within that range, your friction I mean sorry, your velocities are going to be you know, regular.

you're not going to get you know, 10 feet per second velocity if you stay within the right friction rate. so it's gonna kind of self- adjust itself for most of the time. Um, you know. And then on my presentation that I did about friction rate on Friday at the Symposium I compared three duck systems I compared a flex duck system um a round sheet metal system and then a rectangular trunk with round rectangular uh branches coming off of it and um the the.

The flex duck had by far the the highest friction rate which is actually a good thing. You want a high friction rate which sounds really weird because a high friction rate means you have less equivalent lengths in the system and a and a high friction rate will actually uh, result in a smaller duct because you can get if you have a high friction rate, you have a higher budget to work with so you can have a smaller duck to to use up that resistance. but um, it was kind of interesting to see that and I just I've all 99.99% of my designs have all been Flex 100% flex and they've all tested out wonderfully. They're like better than what we designed to.

They always test out better in terms of air flow and static pressure and everything. and I think part of that is because Flex duck just is more natural for the air flow. the air. it just does more what the air wants to do.

The air wants to be have gradual curves, it doesn't want to make these hard sharp 90s you know and and go perfectly straight and all this other stuff. So um hey Russ I Just put this question up here. Um somebody was kind of asking how can a seven inch duct be twice or can how can it carry 50% more air than than a six-inch duct? Well look at a get a ductulator out and uh, look at 0.1 inch static pressure. Look what a seven inch will give you and look what a six inch will give you and I I have I have.

It's not the the diameter is not 50% bigger, but the area is. You have to multiply by pi times the diameter squared / four. That's the formula for area. You find out that the area is in fact 50% more.

Well, you could just look at it as a ratio of diameter squar. but you're exactly right. It's just do seven divided by six and square it. So 49 divided by 36 I mean you were close for R Us I got 40% about.

but yeah, you're you know? Uh, but but six exactly. So it's diameter squared over the ratios and you don't have yeah? well I'm actually talking CFM I'm talking 50% increase in airf flow not area. Um so at a frate of 0.1 is 6 inches about 75 CFM and a seven inches 110 that's a you know 35 CFM increase. So you've gone some some 75 and you've increased that by 35.

That's pretty close to 50% Okay, because the friction rate goes down a little bit too. No, That's all the same friction rate. It's the same friction rate. No.

I mean I mean sorry the friction Factor back to Moody chart Friction Factor goes down a little bit too. um. and I Want to speak to something Adam pointed out Um, talking about fittings. and I want to clarify some terminology So um, it's absolutely true that almost everywhere in a residential duct system or a commercial duct system, flow is turbulent.

Rentals numbers are probably 25,000 up to 200,000 Um, which means we're way way turbulent all the time. But I think there's a different term that nobody has really heard that describes what people mean when they say I want nice lamin or flow? What they really mean is I I want fully developed flow. So the the picture on the right here shows that you know the the velocity across the pipe is pretty close close to the same and it's slower near the the wall of the pipe. Um, but it's also you know it's symmetrical, right? The top and the bottom of that pipe are are looking the same, but if you were to draw what that profile looks like after an elbow, it all gets squished to the outside.

just like the people in a car. when you go around a corner, it's the same thing. it's momentum that throws it to the outside. And so when we talk about fully developed flow, what we're talking about is that the flow gets uniform and it stays that way.

So after an elbow, it gets all shift off to one side and then it slowly evens out. so it looks like the slide or the the photo on the right where it's It's now it's uniform again. and that's we call that fully developed Meaning if I go Downstream it's going to look the same as Upstream It's fully developed. It's not changing anymore like it does when it recovers after an elbow.

And what's the benefit of that? why would you want that? So in a residential system, um, it actually doesn't matter very much. And in a residential system, the the runs are so short that it's going to be pretty rare to have fully developed flow. Um, but fully developed flow could be important for um, you know, going uniformly more uniformly across a filter, or uh, not getting all thrown out one side of the register boot. Um, if you're talking about, you know, grills that are kind of big, but you know, frankly, even that doesn't matter a lot.

Except that here here's one place. it does matter. depending on how sharp the elbow is, you're going to throw that flow to the outside harder or less hard. So if it's a more gradual curve, you don't get as distorted and you don't lose as much friction.

So when people want a smoother transition, it means means that it's going to become fully developed sooner because it got less Disturbed in the first place and therefore you lose less um pressure because of that disturbance. So what you know that's I think what people are thinking about is how do I lose less pressure and if you disturb the flow less it will. You lose less pressure and it will recover back toward fully developed more quickly. Yeah and I was just going to add to that too.

Real quick. Absolutely U you know I kind of look at it also is you know when you have this rule of saying hey, you don't want these fittings too close together because you may not get the the distribution design uh, the way you wanted it to um by having fully developed flow. when you uh pass a branch or some transition, it'll say sort of recover its profile in a relatively short distance and this also has to do with turbulent flow in a sense that you will fully develop in five to 10 diameters versus is 100 to 200 diameters length. Uh so as I said mentioned before, Lamer flow is kind of like weak so you know, can you skip over a branch if you have really low air flow? Can you even have uh over some transition or or kind of like a a little a joint on the metal duct and sort of trip and separate and basically go right over a branch and not have any air flow come out? That's why I say you can be too big, you could have velocities that are too low.

although practically speaking, we never run into that. One other thing with fully developed flow when you develop quickly, especially with turbulence, is mixing. When you go across a cooling coil or a heating coil that Air Stream is going to be mixed. If you have very very low velocities across a coil, you're going to have streams of cold and hot air.

Uh, and it's not going to be uniformly mixed. and we always want uniformly mixed air when we're talking temperatures and spreading that out. So so how big of a duct would be too big to where you might have a problem? Like where air literally might pass over like a T like a takeoff or something. I I Don't have any real experience on this but just from looking at the correlations and fittings and saying Hey I want to be turbulent and stay in the turbulent regime? Uh, we run into the very low velocities on the branches and kind of like what um Russ was saying too When you have your your point .1 or 8 friction rate.

but as you go down for that same friction rate your your velocities are decreasing and decreasing so it's at the you know, so kind of like at the end of a trunk. If we if we don't have some sort of transition or whatnot, we could have really low velocities. and if we have some last few branches, you may not be able to balance properly when you're having say less than 300 feet per minute or even 200 feet per minute. So if you had a constant, you know, uh, trunk line a 24 by by 8 for 800 CFM and you have 20 branches.

um yeah, say they're all six Ines you know, maybe fours in sixes I'm just giving maybe not too common example for some people, but you could have a balancing nightmare even though manual D and and into to a point that Russ was making to say hey, look, you got plenty of a window but then having to balance it could be the challenge we see now I Wanted to back up for a second because I know Steve was talking a few minutes ago about um, if you have lower velocities or a smoother transition through a fitting, you have less friction or pressure loss across the fitting. Um so I'm just wondering. you know we're talking about fully developed flow in that but when when you have lower velocities in general, you have less equivalent length which is a less pressure drop I would think that would be a good thing. um and I know Matt was just saying hey, what is what velocity is too low of a velocity and going back on that Facebook post and just thinking about all the people that thought they had issues in the past with airf flow based on low velocity I Want to know how common of a of an issue that is? You know what I mean Like how frequently does people run into that? Um I I Find? Well, yeah I I Think A lot of times people think they're having a problem with low velocity and what they're really having a problem with is the blower motor algorithm.

Um, you know ECM Uh, blowers are run by a computer program which is designed to work in a certain range and if there's very, very little back pressure like you just basically put the cabinet in an open room with a you know, duck blown out the top and sucking in the side. the static pressure can be so low that the algorithm isn't in a stable operating range and it can you know oscillate and behave weird, but that probably has more at least I Think has more to do do with the programming of the motor controller than it does to do with the velocity being too low, right? Um, Now one of the one of the topics that came up Russ I Want to ask you about is Comfort Uh, a lot of technicians and people in the field associate low velocity with poor Comfort Like you're not going to be comfortable in a space because your air flow is too low within a duct. Uh, do you want to speak on that for a moment? Yeah, the only, the only real downside practically speaking of low velocity in a duct is you get more conduction, you have bigger surface area, the air is moving slower, and you have conduction. But let's say you had a let's say you had a duct going into a wall of a of a building and 20 feet away of the same size duck was coming out the other side.

Well, assuming no leakage. if you push 100 CFM through that duct, 100 CFM is going to come out the other side. What's happening in between there? Let's say the duct goes from a 4 inch to a 20inch duct and then back to a 4 inch. It's going to go very very slow inside that 20-inch duct, but you're still going to get the same CFM out the other side.

It's just it's while it's in that 20inch duct, it's going very slow and and it's going to have more conduction. So so it's an energy thing. It's an Energy Efficiency thing. But when you when you neck the duct back down to your your register, boot or whatever, the velocity comes right back up and and that what you want is you want velocity coming out your register.

The Velocity in the Ducks really doesn't affect Comfort at all except that you have more conductive losses. The velocity coming out of the register is what's important. So yeah, I hope I Hope that answers that question. Yeah, so let's let's maybe put a finer point on what Russ said because it's really important.

The throw of the register is determined by the dimensions of the register, not by the duct Upstream of the register because the velocity changes when it goes through that different size open and through all the slats. right? And so if I have a a 6x10 register with you know a certain you know I'm throwing left and right. we'll say um and or I've got a curve blade. whatever it is 6x10 register and I'm approaching it with a 6inch round or I'm approaching it with an 8 inch round which is much slower velocity.

the Um: the throw is going to be almost identical because because the throw is determined by the register Grill not by the branch duct. Okay, um Tony I Want to ask you? you know? back to the original topic we we were discussing right before we went on: Russ mentioned conduction and maybe he's talking a little bit about um. heat transfer through the duct. Somebody mentioned that in the comments as well too.

I Know Manuel J has a a way to calculate duct loss and duct gains through through the surface area. Can you dive in a little bit to the the whole velocity aspect and what the what? Real life? You know what happens in real life versus Manuel J Well, you know that there's a lot of a different scenarios. It's it's kind of. You know if the question is, hey, if I have the same CFM and I double the diameter, what would be the heat transfer Dynamics Uh, going on or versus just saying the same velocity but I'm just doing double the surface area with that same velocity and same temperature difference.

So you know I got a lot more volume of air moving through it. Um, but um I Just want to mention that manual J is also a simplified and estimated duck load for you know, I Know a lot of people sort of believe it's super accurate. Uh, you know, radial perimeter, interior, radial trunk and Branch trunk and Branch perimeter? Um, they're just estimates on area based on what you select. I Kind of like to be conservative on my duck load, so you know depending on I I I See, people do homes and they love to put all the supplies at the perimeter.

This is duck work in a 150 degree 140 degree attic. Um, and I'm like, oh geez, I I design my system to be much more shorter runs, but it's kind of a way to just sort of cover my butt. Um, and stuff like that Manuel J Also assumes the attic temperature is always the same. If it's a summer time, it's uh, 140 135 degrees.

uh for a dark roof vented attic. Um, if um, in the winter time, the attic temperature is always the outdoor. uh, dry bulb temperature if you're in Zer degree weather or or a 10 degree design winter, that's always going to be the temperature. But the reality is when you have more or less surface area of duct work uh or just say more or less duck work in the Attic There is some people call it a regain but basically saying hey, look this you know 50 55 degree Supply Air duck work in my attic.

It's absorbing some of the heat. so it's going to be by absorbing some of the heat from the uh roof deck and you've got ventilation going on. You're also reducing that attic temperature. So if you have double the duck work in an attic, you know you should see a five to 10 degree temperature reduction in your attic.

And same thing in the winter time you've got hot air in in Duck work. Uh, if I didn't have any duck work in the vented attic I'm close to the out temperature I'm actually a little bit uh, warmer than the outside temperature. but once I throw hot duck work in there I I'll go up another 10 degrees in the attic. and if I have more surface area I have you know more so then it? And keep in mind the duck load.

whatever you're emitting is that you know, uh, look at that duct. You know insulation in indoor outdoor film with the duct itself. look at that as Pete transfer through a wall. Um, you're changing the conditions.

When you're changing the Dynamics in side, you're not only changing the delta T you're not only changing the surface area. Uh, you're changing. Yes, the conduction or or or convection. I Always call Uh, this my take I I Always call convection is conduction.

Convection is conduction between a fluid and a solid. So if you've got air moving across a solid surface, that boundary layer is also a thermal boundary layer fluid boundary layer. That that's a it is conduction. It is conduction.

But it's a conduction from a fluid to a surface. And it's called convection because of the two different states. now. um, anyone? I mean this goes to any three of you that want to answer.

but uh Tony mentioned air film and I know that's like a super weird thing to wrap your head around like the first time I heard about air film. um was talking about Manuel J and air films on the inside and outside of walls and that and I guess it has the same thing on Ducks So um, if any of you want to speak a little bit to that and just give a you know everyone an idea what the air film is and how it impacts a load I can take that one. So in when fluid is Flowing across any solid surface, there is Um in uh, fluid dynamics terms, what we call the boundary layer and the boundary layer. You know.

So even air has viscosity. So you can think of it, you know it is actually thick like honey, just way less thick like honey. And because air has some viscosity, the molecules that are right up against that solid surface, they stick to it. And when uh, so the molecules actually touching the inside of the duct are stuck there and they don't move hardly at all.

And the one next next to it because there's some viscosity, it only moves a little bit faster. and a little bit faster. and a little bit faster. So that diagram in Tony's first slide showed.

You know that there's this area right next to the edge of the duct where the velocity slows down. Yeah, so this one here. so you can see that area like the top, you know, tenth of the diameter, where the velocity right at the pipe wall is zero. and then as you get even a little bit toward the middle of the pipe, it goes way up really fast.

So that's called The Boundary layer. Um, when fluid is flowing across a solid object and the thing that that causes in terms of heat transfer is it causes a fluid film because in order for heat to get into the main stream, it has to pass through that boundary layer or pass through that film that's not moving very much. And so when you do a convective heat transfer calculation, there's a number that's called a film coefficient. And that film coefficient is the number that you use to characterize how thick and how slow or fast moving that film is because that will act like insulation.

because it's not moving very much, it's just kind of sticking there. And you know, uh, the R value of air is great. If it's not moving, it's quite so. Yeah, So right.

That's how that's how Fiberglass Bat works, is it just holds the air still. It's actually the air is the is the insulating material, not the fiberglass itself. Yeah, glass is actually pretty good conductor of heat. It's the air that you want.

The glass just keeps the air from moving. That's the best explanation. I've heard and you should write a book because it took me a while to to get past this when I was learning this in school. I was like what you know, a thermo boundary layer too.

You know? Yep, a thermal boundary layer and a fluid boundary layer. So the so lower velocity will actually give you like a more insulation due to this yeah uh, air barrier. what's it called again yeah. Air film.

Yeah, air film or the boundary layer or the you know yeah fluid film. And I was going to say if if you look at a whenever we talk uors of a wall um U factor is the Assembly of our values and and let's just say you had a an R10 board and an indoor film of one and an outdoor film of one I'm just making up a number in an R10 board you would add up those numbers and say um you you would reciprocal the H and say well okay, reciprocal of one is one So my U factor is uh, the Assembly of RS which is 1 plus 10 plus one and 12 reciprocate that. um and so one divided by 12 is like a 0.09 u Factor assembly of a wall Everywh, you know where U factors uh look like you know you're typically less. One same thing with the duct.

Um, you've got your insulation of R six or R8 and then you've got that outside film and inside film and when that film is decreased because of low velocity, your assembly your duct assembly U Factor has dropped so you you will have um, the the area may be bigger but the U is less and the delta T is less. So you always got to do the math. So going back to the original point is it's not double, you know Manual J is a bunch of simplified assump so I don't like to mess and I think it's not worth uh, going and calculating all your duct areas and and then are you using inner area or outer area? I believe manual J which they never tell you if it's the inner area or outer area. You know the difference between the Inner and outer surface area could be 20 something percent if you're R8 and and all that stuff.

So I just be conservative on my duct r with it. So this is yeah, this is kind of Eric was getting at the same question I'm getting at Does this mean that the lower duct velocity is actually going to to help you? Is there a point of diminishing your turns with this does? Is this is this whole idea of this extra layer of air? Um, you know, is this more hypothetical Or or is? or? or could you actually use this to design a duck system? Sorry go I Wouldn't use it to design a duct system because we're talking about. um, we're talking about effect that go opposite directions. So yes, the film coefficient will be lower meaning you transfer less heat, but also the area will be larger and so you have to figure out which of those two effects that are going opposite directions dominates.

and I think it's the larger area dominates and you will actually get more heat transfer through a larger duct. I Me too. I I Agree As a general rule: If Ever I Probably mentioned before I'm I'm just kind of conservative. The Manual J Duck Load I think I Want to say it's typically 20% of the floor area is a duct surface area and and we just take it, we run with it.

I may be off by 20 30% You know? Uh uh. There's ways of you know when you are doing a full top to bottom uh Jsd, you know I just try to keep my duck work minimal and simple. Uh which in reality would have say smaller duck loads just from having much less duck work is it is a software actually capturing that. The answer is no, it's really not.

You know it's You know, these are really estimates, you know. I I Know we try to be as aggressive and detailed as we can, but we don't even really know the duct surface areas. We really don't know the Supply Air temperatures until we pick the system and do the duck work. And you know Manuel J They do say okay, you could pick your heating temperature but they assume it's I Want to say they never told me it's 55 degrees Supply Air Temperature So all this stuff that you're it's good to think about or it's good to play with a little bit if you want to get nerd out on that.

and when I do I I lose my mind and I say you can't win. You know because you could just keep on all day going back and forth and tweaking your loads, tweaking your duct areas and and then you never really get anywhere and it's just U I Say this is a plus or minus 20% world and and if I'm within 10% I'm I'm kicking butt. Um, so yeah. I Don't worry exactly what Stephen was saying too.

I I Don't worry about it too much, just you know it's great discussions. Yeah, so it's back to what Russ said. We don't size ducts to 6.2 in and 8.4 Ines and because of that, we're always going to come out on the large side. Uh, you know, so it it doesn't really matter if we have errors that are.

You know if you're off by 20% of the duct loss and the duct loss is only 12% of the load, so you're really only off by 1.2% It doesn't matter. You know you're going to use, you know it's it's way in the you know the range of the the difference between your seven and your eight inch. You know that's something I was goingon to ask Russ like what do you? you've done? Probably, you know, arguably more loads than almost anybody, right? Um, what do you normally see? Um, knowing that you design systems with the duck work predominately in an adct. What's a normal duck load? Like what do you normally see compared to the entire load of house? Um, most of my career was doing new construction so it's going to be pretty low.

Um, it's going to be. You know the the the total duct loss is going to be you know, 12 14 16% of the of the of the cooling load. Um, it can be less than that. Um, but uh, but that includes leakage which is a big chunk, you know.

and so you know we're wor we're worrying about and leakage could could be the majority of that number. And and you know we're woring about a little bit of conduction loss and then people aren't even testing their ducks for leakage and they're stressing about conduction losses. you know, So that's it's. really lost in the noise.

but I was I was going to say the exact same thing that that Stephen said. You know in in quick model, you can actually calculate the exact surface area and you can plug that into the energy gauge loads instead of using manual J's assumed surface area and you can plug that in. and I did that one time and you know it took me four or five minutes to calculate it and plug it in. And it was.

It was. It made a 10% difference of a number that was 10% of the total load and so it was. It had 1% impact on the entire load to do all that work. So yeah, it wasn't worth it.

but you know I think all three of us being Engineers we would do that to find out. Of course, yes, you got to. Well, you guys can do. You guys can do it so that you can tell us it's you.

Don't need to do that. We did it. You don't have to. We did it.

It doesn't matter, You don't have to. It's fun to do. It's fun to do. I You know sometimes I think it's fun to do and just because.

Now we know and we we share it and you know and we may have done different tests and you know and by the way, when I do my duck. loads in addicts I assume some leakage and that plays into my stuff too. It's fun, you know? So anyone who likes to play with that? Yeah, so I have a I have a saying that you guys have probably heard before. but I I'll say it to everyone here is you know, don't waste time splitting hairs because our job is to shave heads right? We're trying to get.

we're trying to get as many load CS done as we can and get these houses going and do it right and just do it right. Do it. Just do it. you know? and don't stress over these these little things.

I've seen people freeze up because they they they didn't know what to put for the color of the roof like I don't know. it's kind of a medium gray but the choices are dark and light and I don't know which one to pick and it's just a pick one to go. you know so well and I would say you know if you if you can get a hold of one of us to help you answer that question great but if not, just try it both ways. In your software it it'll take you 10 minutes and you'll know.

Oh, it makes almost no difference in my load count. Just like vaults, vaults, and everything too. vaulted ceilings versus flat. it's like I I don't I don't even see a percent typically and people just think that fault's such a big deal, you know? And yeah, yeah, I think you just kind of have to.

you kind of have to do a few to to see that right. Like and like and like Steve said, just kind of play with a few of them. So I did have a question. where do you think that these typical numbers come from? You know, in in manual D um, what is it? Uh, 500 feet per minute on the return and 700 on the supply side? You know, are those just kind of like yeah, why did why did we end up with this I Think that's kind of an average number based on an average friction rate.

You know to pass a certain amount of CFM through a certain size duct. Um, you're G to be somewhere in that range. Um, and that's just average and obviously if it goes, if the velocity goes up the the the equivalent lengths get longer and if velocity goes down so you know some ducks are going to be more, some ducks are going to be less. It just all kind of averages out.

Hopefully in the end, if you're doing it right I think I've heard Doug or I've heard not Doug I've heard Ed say that um, there's uh, duct noise as part of it too. Once you get above a certain velocity, even if you had, you know enough um available pressure loss to do a higher velocity. Now you're going to have noise problems and noise complaints and so um, you know you just keep it under that. Do you guys have like a a general idea or recommendation like velocity wise when you're designing a system? Or let's say somebody is downsizing a system and they're like terrified.

The Ducks are too big, Like what is? Is there something you would just right off your hip say that's too low velocity? You got to stay away from that. I I Think that if you do manual D correctly. Um, and you decide that you're going to be super conservative and you're going to go up one size bigger than Manuel D says you're going to get a really low static pressure loss, but everything's probably going to work fine if you go like two sizes bigger or three sizes bigger. I think you're going to potentially start to have problems of too too high a duct uh, duct gains.

So if you put like a 12 to every bedroom in the house or something like that, yeah when you when it calls when it calls for a seven and You just you're going to put a 12 to every bedroom Yeah that you know you might be having problems I I was gonna say too well um you know although I'm I'm kind of. you know when it comes to trunks you know I'm typically maybe on if if if we're talking four or five tons sure I'm looking at and I do a th000 feet per minute. uh I got plenty of information that more noise comes out of grills by far than the duck work even if you're 12 00 feet per minute. uh depending on what standard you're referencing I know D has the 900 feet per minute in trunks but I'll take it to a th000 and and I'm not.

you know that's a 24x 12 rectangular tunk trunk with 2,000 CFM When I'm doing branches and say Flex I I tend to be around the 350 and I'll tell you why that all sort of f Falls in nicely with the 08 friction rate I think of uh Russ your your card a brilliant card I mentioned before threein duct we need to squeeze that in. Um so so when you're around 350 my issue with manual D and I have a few issues with so don't shoot me guys and this is just my opinion. just my opinion. No, we we like opinions.

I I Tell people Manuel D is not a duck design guide I I always say you're the designer I Don't think Manuel D tells you what to do I Think you do the design and you follow some guidelines if you want to do a transition or if you're going to do the spider or the radial I Don't think there's anything manual D tells you which one to choose I think you make that decision and what to do. Um, the equivalent length method. Uh uh. it's exactly what uh was mentioned before Russ it's it's it's for the budget what the manual D does.

In my opinion, the only thing it really does is it determin. It's a way to determine the critical path I Want people to know that it's a way to look at all these different fittings. although the softwares do that for us is they find the critical path and we based on the fan pressure, we really we're coming up with the maximum allowable friction rate I Want to say that again, the maximum allowable friction rate. maximum allowable friction rate means smallest ducks and even though you're going to have a friction rate of 0.12 or 0.18 or whatever, the and I even go above 0.18 you know is as long as you know friction rate has nothing to do with manual d By the way, it's just it's that's duck sizing calculator stuff.

Um I'm always going to pick a 0.1 or 0.08 I'm always going to do it just for low pressure, just for balancing and just for margin. Uh, if I get a friction rate 07 or 06 I will use 07 06. I can't exceed that friction rate. But if I get a 0.12 or 017 or I'm going to go back in default to the 0.1 or 0.08 it's going to make for a much better balanced design.

And I'll say this about my issue with manual D is it never makes a balance design. As you guys are all saying is, you could use your 0.1 friction rate. but if you get a 6.2 you do a seven. If you get a 7.1 or 7.00 Z something, the software is going to HIC up and give you an A So of course you're going to have plenty of margin.

That's nothing magical about the manual D It's really just saying I'm going to size on a friction rate calculator this and if it's bigger I'm going to go always round up. but that doesn't give a balanced design. you're still on the hook for balancing. Why? I like the 350 feet per minute for the branches and if I've got an air handler.

but let's just say I got a trunk going one way and a trunk going another and I look at my friction rate you know say you know 0.1 actually I Want to size my trunk branches such that it'll be at a little bit lower friction rate. but I'm going to look at the two velocities I don't want to have a a 300 feet per minute going I'm sorry 300 CFM going one way, 400 CFM going the other. If I use a 0.1 friction rate, I'll probably have the same size duct, but maybe I'll make one. Uh, you know I'll bump up a size on the 400 until I'm kind of looking at equal velocity and there's plenty of resource that says when you size for equal velocity Trunks and equal velocity branches, you will have a much better uh balance design from the start.

not perfect, but a much better balance. I'll give you an example: friction rate. What? What is friction rate? Friction rate is really pressure drop per 100 feet of duct. but you guys know when you guys do your total equivalent lengths, you're looking at a few hundred feet, but you probably only got a 100 feet of duck length so it has really nothing to do so.

I You know whether I got a flex running 25t or one running five, the resistance just in the straight duct is negligible compared to all the fittings and everything it had to go through. So if I size things at roughly that 350 feet per minute on the branches and if I've got more than or if I'm going to have one long trunk and I'm going to transition in the middle say you know I I think kind of what determines how many transitions is where I could fit in between some Branch space. But I want to neck that back down to the original velocity that I started out with. So I'm trying to make a constant velocity trunk and then all my branches are going to be a much lower velocity because say for the same friction rate of 0.1 or 09 or 08.

Um, those are going to be around the the the 350 feet per minute. the 350 feet per minute for 8 inch duct is going to be a much lower friction rate than the 4 inch 350 feet per minute which is going to be 30 CFM or 25 CFM So that's my take is I'm just kind of like saying well you're really the designer, you're just finding the friction rate you could use. The reason I won't go above 0.1 in in residential systems is liability. There's plenty of Engineers if there's issues and make commercial engineers get hired and they want to look at your duct design.

If they're like oh, they didn't use a 0.1 friction rate this is all wrong. You know we always say just don't pick the 0.1 or just don't go with something random. You got people that'll hold you and never do a load Cal in their life never did a manual D in their life. but they'll just say every Reon rhyme and reason why you did a duct design bad because you used 0.13 friction rate.

So I stay away from above 0.1 And and then you guys know your trunk and Branch or or designs if you're above the 0.1 for a manual D you've got low resistance. Go ahead and use your 0.1 or 08. Yeah yeah to get to to get to a friction rate, you know, like a 0.13 though I mean you're talking about like there's no fittings. um and you know your equivalent lengths have to be really short and you have to have a lot of available static pressure.

So so I had a I had I had a unit like that that I MISD designed basically um I I forgot to add the coil pressure drop in and so and then I was using quick model and I and I said I said Auto siiz the Ducks and uh they were like all 4 inch Ducks everywhere. um and it it was kind, it was. It just looked really funny. You know I mean obviously I was like oh my gosh, something is wrong somewhere.

um because they were all tiny. you know. um you had five inches of available static pressure instead of 0 five Ines Yeah Yeah Well yeah so I mean and and one of the things I've noticed is you've got an air handler now that might do point I mean so it'll It'll give you the correct CFM up to 0.9 inches of static pressure. and and if and if all you've got is a filter and duct work, you could actually have some pretty small Ducks So so at that point you do have to size Back Down based on velocity.

That was kind of what somebody recommended to me. Yeah, yeah. and just so you know, when when I've got to the 08 and even with a half-inch air handler, it's been where I had the return right by of the air handler and it was a big ceiling area and and it was actually a F ton and I think I just had. you know a four 12inch Flex lines coming right off the plenum.

So and then when you do it that way you know you could have a really high friction rate and stuff. but I know it's going to be high velocity and and that's the thing about the ACA wedges. You could exceed that 900 feet per minute if you use 08 on a you know 2000 CFM thing. but the software will kind of prevent that from happening and give you a larger duct anyway.

so people may not know that when you've got those velocity limits built into the software, it's not going to let you go above that velocity limit anyway, so you may not even know that that's already being taken care of for you. So again, you're designing for a much lower friction rate without even knowing. Yeah, and like a a higher friction rate once you exceed 0.1 A lot of the times, Um, you'll be over the 900 feet per minute velocity, right? Um, Which once you're over that velocity limit like all bets are off when it comes to the equivalent leg fittings. a manual? D you're you're You're instead of the fittings giving you a little bit of wiggle room.

Um, you're actually under underd designed. Or you know, the pressure loss is uh, underestimated if you will, because the fit the fittings are assuming a certain Uh velocity. So I think there's some confusion about that that the table says that these are at 900 feet per minute. but it also says in the manual that the definition of an equivalent length is how many feet of straight duct at the same velocity? So it's that at the same velocity that matters.

So um, I You know it's not like they drop off a cliff where if you're at 1100 feet per minute instead of 900 F feet per minute, it's not like your Equ links are going to double or anything like that. They might be slightly higher or lower, but they don't change dramatically as your velocity goes up a little bit. So I played with um and I I know I I called you Tony on this the one time because like the one day I had like notebooks out all over and I'm starting to you know I was actually playing with Matt's uh duck design that he he did a duck design for HVAC he did a tech tip and it was for a slim duct. um, like one of those low static mini split air handlers and um I was trying to figure out what the actual equivalent lengths are based on the velocity since his velocity was so much farther under the manual.

D and that was like they are less. but when you start using, there's like a formula in manual D that you could use to calculate what the actual Um equivalent lengths are. but it's like an iterative thing and it's like a wild swing. You do it once and it's like really really high and then you do it again.

it's super low and it just keeps swinging back and forth. and I did it like five times I'm like yeah, I'm done I'm not doing this ever again. This is stupid. Well, you know it tries to undo what it did.

Uh, you know Br's equation half over V^ squ time K that K is your flow loss coefficient. So all these components basically have a k and by the way, K could vary with velocity. Um, so not just the the fitting itself could have a pressure difference because the velocity of the fluid, but also the flow coefficient is a function of aspect ratio and area and incoming in and outcoming velocity when you're talking Transitions and expansions. So whenever I did fluid modeling on on the commercial scale and I mean for other types of jobs not HVAC everything is flow loss, flow loss moding.

I I I Think that's something just because I Think everyone's using software anyway. I I think I Still think it would be a better way to to go because at the end of the day I Still think you're going to have margin? You're going to find that friction rate you're always going to be upsizing. I Think manual. D They call it the equal friction rate method.

By the way, there's all these different duct design methods. An equal friction rate method is never equal friction rate. Really what it means is um, I'm sorry. it's never really equal friction down each path.

It really means the same friction rate that you're sizing every duck with. But that's not even true because if you got a 6.1 you're a seven. Or if you got a 8.2 you're a nine. and trust me, those don't automatically work out to that was a 8 and that was a 08.

But once you at the end of the day, that actual duct is a you know, 0.07 something or 0.06 something. So from a pressure drop standpoint, it's quite a bit off. Sure, it covers from an equivalent length and um, you know. But then the problem is, uh, you know, can you use static pressure in the field to troubleshoot something? No, you're not getting that stuff from manual D And and you never will.

Um, and then when you're trying to be a little bit more and then the problem is with those you know, 0.2 inch, you know, ducted minis? Um, you know you're not going to have you know, 900 feet per minute duck work, you know? But trust me, you know a 50 feet or or even 20 feet of equivalent length could make you your break. you um on on these things with these low pressure heads. you know? So it's kind of like and by the way, high velocity. Ducks They kind of take you away from the manual D2 and by the way, I think I Had the chart where you're looking at friction rates that are like 0.005 for a 4in flex.

you know? Uh, 25 CFM Pretty common by the way. but also for your high velocity ducts, your friction rate is six. It's six. 1,400 feet per minute or 30 CFM out of a 2-in flex.

You know the flow coefficient method. That's everything that takes care of everything. And if we got software to do stuff, let just say it seem. Yeah, it seems like Manel D was written so that it could be done by hand and it wouldn't be iterative.

and but I don't think anybody's doing these by hand and so I think they they might as well just use the method that Ashray does. Yeah, I mean the software could do it easy. And and I I know? Uh, you know some softwares do it. you know, in the maybe in the commercial? Uh, even? uh I Think you know on the commercial stuff, they they do it that way.

They don't do equivalent length and and equivalent length. Um, well yeah. anyway so I won't keep on talking bad about it. Sorry.

use it. Russ What's Russ What's your perspective? Um, keep it simple. stupid. Uh, that's my.

that's my motto in life. Um, you know it's. Let me just say kind of this about you know Tony mentioned balance and I've tested a lot of problem houses and um I've I've measured the air flows I've figured out the target air flows I measured the air flows and I would get I'd measure a room that was 25% low of what the target was then I'd to measure another room that was like 60% low. The homeowner didn't complain about the 25% they complained about the 60% So the other thing to realize too is is these: Target air flows that were calculating for these rooms.

They're only right for a very short period of time during the day as the Sun Passes over that house. it changes dramatically. So the amount of air you need in one room in the morning is very different than the amount of room air you need in that same room in the afternoon. Why are we shooting for such a precise number? You know, why are we worrying about it? So we just need to get that air there and and have good air flow throughout the entire house and get good circulation.

This all boils down to proper equipment sizing you have to do a good load C and sizey equipment so that it doesn't short cycle if you have these nice long runs. All these imbalances are completely hidden in these Long Run cycles. but if you have short Cycles you get runes that are too short short and it just kind of ratchets away. It gets worse and worse and worse every time it.

Cycles it loses a few degrees and loses a few more degrees. But if you have, if you have good long run times, it's all hidden. you can. It's so forgiving when you have good long run times, so that's kind of.

That's kind of my. I'll say one thing to support what Russ just said. I put a Data Logger in my shortest duct run in my house and my longest duct run in my house and I think my shortest is like six feet long and my my longest is probably 60 Fe long. And not only so, when the furnace goes on or the air conditioning goes on, it takes way longer for the air coming out of the long run to get up to temperature than the short one which is going to surprise nobody.

But also it never gets to the same temperature even though my ducts are mostly in conditioned space. Um, actually all in conditioned space. Um, But you never get up to that same warmest temperature for heating or coolest temperature for cooling. Um, just because your your duct losses actually matter when you know when you're comparing a really short to a really long duct.

And so like Russ said, if you're if you're short cycling, it only makes that problem worse. You know because it it might take my long run might take 8 minutes to get up to temperature and my short run is up to temperature in two minutes. So what happens if my cycle is five minutes? my lawn one never gets up to temperature. Yeah, it's It's seems like you could like.

for the average contractor, let's let's let's let's land there. What? What does this all mean for the average contractor? I Think I Think that's kind of what we need to wrap it up with. if you don't do anything crazy like run a 10inch duck to a bathroom. Um, you're probably okay if you size your.

If you size your equipment correctly, you're you're probably fine I'd say I My sentiments are are similar. Trust Manual D It works and ducts that are too small are a much much much bigger problem than ducts that are too big. Yeah, I'll uh one thing I kind of always mention about load calcus is I I I think we rarely ever under siiz I think the only time you ever have to worry about undersizing is when you're doing some super ins ated home and then sometime later way after your job is done and down the line of the project the homeowner is like wow, all this insulation is killing me. These sip walls I'm going to go back to you know, r13 2x4s or concrete block R5 board and if they don't, if that information doesn't make its way back up to you, that's when you undersize a system.

You could oversize a system and and not exactly get mold or have a big liability. So I kind of look at it as I think the biggest thing to worry about is is the distribution design for Comfort just say Comfort I I think the load C is the most important step but it's also the first big step you got to do. But one thing I just kind of not is I kind of say hey, look do your manual D's don't exceed that friction rate if I'm above the 0.1 I like the 0.008 for sizing things, it's a Uh for Energy Efficiency reasons and and consistency and in air flows. One other thing is I try to make your ducks consistent.

You know I know sometimes software will give you a 4 in for um uh, a 22 CFM and give you a 5 inch for a 29 CFM just make it a four. You know, try to make all your 30s like a Ruster card I That way you don't get questioned on um, you know what should that CFM be If all your duck sizes are right and you've got a typo in a CFM anyone using cat or whatever, you know, why am I looking at 80 CFM but a 5inch duck that doesn't make any sense, You know you could explain it. so you know, try to keep your fours 30s, 550s, 670s, 700. Stuff like that too.

You'll have consistent designs uh, and minimize error in the field. And I do a lot of homes, you know I know you all you guys do too. But I'm probably doing three to five small homes a day. And one thing about doing small homes is you get a lot of them done in in little time and you know I'll have a handful of complaint homes, you know and then trust me it, it's making me look bad when someone sits in their little powder closet says no I I need a supply vent here because I could tell there's a temperature difference I was inside the home and there was no temperature difference but I swear we were both sitting on his toilet and and I didn't feel the thing and he did.

he did. That's just enough to drive you crazy even though I don't put one in there but I just try to cover my butt I just don't want things coming back to me. that's on and I hate things coming back to me so just put a register. just got a hold in the shrock, put a register in the ceil that's three in flexus for right? I I'll add one little one quick tip about undersizing how how it actually doesn't happen I Think people C system is not able to keep up and I think that I would bet that more than half and probably 75% of systems it's because of duct leakage to outside, not because it's under size.

Yeah, well, thank you guys. Uh one thing I Just realized as the amateur that I am I am no Brian or I didn't introduce everybody so I'm Adam muff with H HVAC School Matt Bruner also with Hbac School Stevenh Rogers with the energy Conservatory Russ King with quick model uh, coded energy and Tony amadio PE load Cals and true loads. Thank you guys! I Appreciate you guys.